Birthed on YouTube during the COVID-19 pandemic spreading conspiracy theories about vaccinations and a cabal of global elites, the party, Sanseito, is widening its appeal with a 'Japanese First' campaign ahead of Sunday's upper house vote.

And while polls show it may only secure 10 to 15 of the 125 seats up for grabs, it is further eroding the support of Prime Minister Shigeru Ishiba's shaky minority government increasingly beholden to opposition parties as it clings to power.

"In the past, anyone who brought up immigration would be attacked by the left. We are getting bashed too, but are also gaining support," Sohei Kamiya, the party's 47-year-old charismatic leader, told Reuters in an interview.

"The LDP and Komeito can't stay silent if they want to keep their support," Kamiya added, referring to Ishiba's Liberal Democratic Party, which has ruled Japan for most of the past seven decades, and its junior coalition partner.

Kamiya's message has grabbed voters frustrated with a weak economy and currency that has lured tourists in record numbers in recent years, further driving up prices that Japanese can ill-afford, political analysts say.

The fast-ageing society has also seen foreign-born residents hit a record of about 3.8 million last year, although that is still just 3% of the total population, a tiny fraction compared to numbers in the United States and Europe.

Kamiya, a former supermarket manager and English teacher, says he has drawn inspiration from US President Donald Trump's "bold political style".

It remains to be seen whether he can follow the path of other far-right parties with which he has drawn comparisons, such as Germany's AFD and Reform UK.

Yet the ingredients are there, said Jeffrey Hall, a lecturer at Tokyo's Kanda university who has studied Japan's right-wing politics, pointing to their online following, appeal among young men and warnings about immigration eroding indigenous cultures.

"Anti-foreign sentiment that was considered maybe taboo to talk about so openly is now out of the box," he added.

With immigration emerging as a top election issue, Ishiba this week unveiled a new government taskforce to fight "crimes and disorderly conduct" by foreign nationals and his party has promised to pursue "zero illegal foreigners".

Polls show Ishiba's ruling coalition is likely to lose its majority in the upper house vote, in a repeat of elections last year in the more powerful lower house.

While he is expected to limp on, his government may have to broaden its coalition or strike deals with other parties on policy matters, analysts say.

Kamiya, who won the party's first seat in 2022 after having gained notoriety for appearing to call for Japan's emperor to take concubines, has tried to tone down some controversial ideas formerly embraced by the party.

His election manifesto, for example, includes plans to cut taxes and increase child benefits, policies promoted by a raft of opposition parties that led investors to fret about Japan's fiscal health and massive debt pile.

While Sanseito is the latest in a string of small far-right parties that have struggled for a foothold in Japan's staid politics, its online support suggests it may have staying power.

Its YouTube channel has 400,000 followers, more than any other party on the platform and three times that of the LDP, according to socialcounts.org.

There are still hurdles. Like right-wing parties in the U.S. and Europe, Sanseito support skews heavily toward men in their twenties and thirties.

Kamiya is trying to widen its appeal by fielding several female candidates such as the single-named singer Saya seen as likely to clinch a seat in Tokyo.

Earlier in the campaign, Kamiya faced a backlash for branding gender equality policies a mistake, as they encourage women to work and keep them from having children.

"Maybe because I am hot-blooded, that resonates more with men," Kamiya replied to a question on the party's greater appeal to men.